Not sure what you're trying to do here; Any decent balance should have some zeroing function, so just put the foil (or any weighting board you are going to use) and zero the balance with it before weighting anything.
And your calculation is wrong; if 1mm square is assumed to weight 1 mg, then a 50mm square (or 5cm square) should have a surface of 50 x 50 = 2500 mm2 and should assumed to weight 2500mg, not 50mg.
If on the other hand you are looking to calibrate your balance, balances usually comes with calibration weights. If not, then coins can be used as calibration weights. The UK penny coin weights exactly 1/8 of an ounce, that is around 3.4grams. similar information can be found for coins of other countries I would presume.
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